Centrifugal separators for separating solids from liquids are well-known. They are used in irrigation systems to remove sand and debris from irrigation streams, to remove particles cleaned from parts in a parts-cleaning system, and scale and grindings from metal-working systems, to name only a few examples. A family of such separators is exemplified by the showing in Steven D. Ford U.S. Pat. No. 5,368,735.
Separators of this type provide a cylindrical separation chamber into which a stream of liquid (usually water) is injected. The stream carries entrained solids such as sand, grit, metal particles, and debris. This stream is injected into the separation chamber with a substantial tangential velocity. The stream flows helically downward to a spin plate, where it reverses its direction and flows centrally upward to exit the separation chamber at its top.
Because the entrained solids are denser than the liquid, they migrate closer to the wall of the chamber. Near the spin plate at least some of the solids continue to move downwardly and radially out through an exit port instead of flowing upwardly with the exiting liquid. It is customary to let the separated solids settle in a lower collection chamber, and then periodically to purge the collection chamber, along with a burden of liquid.
This is more than a nuisance, because if the collection chamber is not drained frequently enough, the collection chamber will be filled with solids. The separator is then incapacitated even though it continues to pass the liquid stream. Even occasional drainage through a stopcock valve entails this risk because of the risks of unobserved additional burden or of neglect. The supervision and inherent inefficiency and wastefulness of this arrangement is well-known.
It is an object of this invention to provide a drain for the solids which is always open, but which permits the flow of solids through it at a rate that is consistent with the accumulation of solids in the collection chamber. When properly designed, the drain conduit permits the steady passage of a thickened stream with a minimal discharge of liquid with the solids. This prevents accumulation of solids in the collection chamber and with minimized liquid loss. Periodically, lost liquid will be replaced, if in a closed system. The system need not be shut down to remove the solids, and loss of fluid is less than for conventional drainage.
This invention provides some rather unpredictable and surprising results. When the drain conduit is provided, it actually increases the separation efficiency of the entire system, because by continuously removing the solids it encourages the smaller particles to enter the collection chamber (sometimes called a "purge chamber") past the spin plate, and are less likely to be carried along in the effluent mainstream.
Also, and especially when the drain conduit is formed in the shape of a coil, which reduces its height relative to its length, the tendency of the contents of the conduit is to remain as a rather more consistent mix, with the solids dispersed evenly through the length of the tube. This is a significant advantage when the system is shut down, because a solid plug is less likely to be formed. When one uses a shut off valve to control the purging, a solid plug is likelier to form after the drain port is closed, and this can make a plug which must later be removed. This invention does not involve this difficulty.